I constantly come across invalid email addresses that have been uploaded by some of our marketers. The biggest issue are those email addresses that include a space after it (still not sure why Marketo cannot remove any extra spacing during the list import process - if you agree,
vote this up here). I've noticed other email addresses that contain an extra period, for example. Unless you physically open the lead record - and see the red squiggly line - there's no indication whatsoever whenever viewing leads in list view. It would be incredibly helpful to include an icon of some sort next to each lead record to indicate an invalid email address is being used.